Here is a little mystery that I was hoping to crowd-source an answer to. Below are some magnified views of a crystal-like substance that spontaneously began to accumulate on the inner surface of a glass window (UHV grade borosilacate) that is part of a vacuum vessel in which plasma experiments are conducted (working gas is helium). The full scale of the image is approximately 2 mm across. The window is on a top port of the vessel so material attached to the inner surface is holding itself against gravity, i.e. it is not just dust. The window is not directly facing the plasma , but gas could flow up a long thin tube and around a bend to get to that port. The vessel itself is mostly stainless, the plasma facing surfaces are either tungsten, copper, or high purity alumina ceramic. Titanium gettering is used on one section but there is no line of sight to the window and it doesn't look like what we normally see when a window gets coated with Ti gettering. Previously the vessel had lithium gettering but that was all cleaned out before this window was installed. So only a trace amount of lithium might be present. Visible spectroscopy of the plasma sometimes shows significant aluminum impurities probably from contact with the ceramic. Based on those possible materials is there any way to look at the structure of this deposit to narrow down what it might be? I realize that if we opened up the vessel and removed the window it could be directly analyzed. However that isn't a practical option for the moment because it would mean shutting down our experiment and wrecking our vacuum. Is there any way to deduce its composition from the apparent structure? #crystallography
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